1. Field of the Invention
The invention relates to page-wise storage systems, in particular holographic storage systems.
2. Discussion of the Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Holographic systems typically involve the storage and readout of entire pages of information, these pages consisting of arrayed patterns representing information. In general, a holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index and/or absorption imprinted into a storage medium. Holographic systems are discussed generally in D. Psaltis et al., xe2x80x9cHolographic Memories,xe2x80x9d Scientific American, November 1995.
Holographic systems are characterized by their high density storage potential and the potential speed at which the stored information is randomly accessed and retrieved. In fact, because information is typically manipulated, i.e., stored and retrieved, on a page-by-page basis, the speed of storage and retrieval compares favorably to conventional magnetic disk or compact disk storage systems. A significant advantage of holographic systems, however, is storage capacity. It is possible for each page stored as a holographic image to contain thousands or even millions of elements. Theoretically, it is believed that at the present time, up to 1014 bits of information are storable in approximately 1.0 cm3 of holographic storage medium.
FIG. 1 illustrates the basic components of a holographic system 10. System 10 contains a modulating device 12, a photorecording medium 14, and a sensor 16. Modulating device 12 is any device capable of optically representing data in two-dimensions. Device 12 is typically a spatial light modulator that is attached to an encoding unit which encodes data onto the modulator. Based on the encoding, device 12 selectively passes or blocks portions of a beam reflecting off or passing through device 12. In this manner, a signal beam 20 is encoded with a data image. The image is stored by interfering the encoded signal beam 20 with a reference beam 22 at a location on or within photorecording medium 14. The interference creates an interference pattern (or hologram) that is captured within medium 14 as a pattern of, for example, varying refractive index. It is possible for more than one holographic image to be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 22 (generally referred to as angle, wavelength, and phase correlation multiplexing, respectively). Signal beam 20 typically passes through lens 30 before being intersected with reference beam 22 in the medium 14. It is possible for reference beam 22 to pass through lens 32 before this intersection. Once data is stored in medium 14, it is possible to retrieve the data by intersecting reference beam 22 with medium 14 at the same location and at the same angle, wavelength, or phase at which reference beam 22 was directed during storage of the data. The reconstructed data passes through lens 34 and is detected by sensor 16. Sensor 16 is, for example, a charged coupled device or an active pixel CMOS sensor. Sensor 16 typically is attached to a unit that decodes the data.
One method of holographic storage is phase correlation multiplex holography, which is described in U.S. Pat. No. 5,719,691 issued Feb. 17, 1998. In one embodiment of phase correlation multiplex holography, a reference light beam is passed through a phase mask, and intersected in the recording medium with a signal beam that has passed through an array representing data, thereby forming a hologram in the medium. The position of the medium relative to the signal and reference beams is changed to allow the data to be stored at overlapping areas in the medium. The data is later reconstructed by passing a reference beam through the original storage location with the same phase modulation used during data storage. It is also possible to use volume holograms as passive optical components to control or modify light directed at the medium, e.g., filters or beam steerers. Other techniques that store data by using motion of the media relative to the beams include aperture multiplexing (see U.S. Pat. No. 5,892,601) and shift multiplexing (see Optics Letters, Vol. 20, No. 7, 782-784 (1995)). Phase correlation, aperture, and shift multiplexing all involve storing holograms in different locations, but with some overlap between them.
As individual data pages are laid down in multiplexing space, there is a limit to how close together the holograms can be recorded without encountering cross-talk during read-out. However, even when sufficient space is provided between holograms, it is possible for cross-talk noise to be introduced into a given hologram during the readout and/or recording of neighboring holograms. Techniques have therefore been developed in an effort to reduce or avoid such introduced cross-talk. One techniquexe2x80x94sparse recordingxe2x80x94is useful for angle, wavelength, and phase code multiplexing techniques, i.e., techniques in which holograms have nearly the same physical location. In sparse recording, holograms that are stored at very close angles or wavelengths to each other are stored in an order distinct from their sequential angular or wavelength displacement. For example, if holograms are to be multiplexed at angles of 1xc2x0, 2xc2x0, 3xc2x0, 4xc2x0, 5xc2x0, 6xc2x0, 7xc2x0, 8xc2x0, 9xc2x0, and 10xc2x0, the holograms may be stored in an actual sequence such as 1xc2x0, 9xc2x0, 4xc2x0, 2xc2x0, 6xc2x0, 3xc2x0, 7xc2x0, 10xc2x0, 5xc2x0, and 8xc2x0. This lessens the cross-talk between holograms. (See C. Gu and J. Hong, xe2x80x9cNoise gratings formed during the multiple exposure schedule in photorefractive media,xe2x80x9d Optics Comm., Vol. 93, 213-18 (1992).) While sparse recording tends to be useful, it is also relatively complex, particularly during readout. Moreover, it""s usefulness, as noted above, is primarily for techniques that involve nearly complete physical overlap of holograms.
In addition to this potential cross-talk problem, degradation of stored holograms can also occur due to local changes in a medium""s refractive index and physical dimensions. Specifically, in photopolymer-based media, photoactive monomers and/or oligomers are selectively reacted to form holograms, and this polymerization tends to induce some local shrinkage. Thus, each successive hologram storage induces additional physical changes in the overall medium, e.g., changes in the bulk index and the extent of diffusion. These additive changes can introduce significant distortion when reading out the holograms, i.e., Bragg detuning of the holograms. In addition, photopolymer media tend to have finite dynamic range, i.e., index change. And, for spatial multiplexing techniques, photopolymer media tend to exhibit non-uniform recording across the medium, thereby inducing degradation of the stored data. (Spatial multiplexing, as used herein, indicates that the multiplexing technique involves changes to location of the medium relative to the signal and reference beams, and that the holograms have some overlap between them.) Advantageously, the holographic storage technique is designed to compensate for such changes.
Thus, holographic storage techniques that substantially reduce problems associated with cross-talk and with physical changes in storage media, particular photopolymer media, are desired.
The invention relates to a skip-sorted spatial multiplexing technique that addresses problems inherent in photopolymer media, including cross-talk and physical change of the medium. Such spatial multiplexing techniques include shift, phase correlation, and aperture multiplexing. Skip-sorted refers to a storage technique in which a uniform background exposure of the photopolymer medium is provided by storing holograms in certain positional sequences. By using such sequences, the problems typically encountered with photopolymer media, discussed above, tend to be alleviated.
According to one aspect of the invention, illustrated in FIGS. 2A to 2C, a first set of holograms 10, 12 are stored in a substantially planar first row such that each hologram just touches the next, i.e., the centers of adjacent holograms are located approximately one hologram diameter apart. (A hologram diameter is distance defined by the intersection of the reference and signal beams in the plane of the recording medium.) After this first set of holograms is stored, a second set of holograms 20, 22 are stored in the same row, but the second set is shifted from perfect alignment with the first set of holograms by an offset distance. It is also possible to store subsequent sets of holograms in this first row, similarly shifted from perfect alignment with the previous set by the offset distance. (Row is used in its conventional sense and indicates holograms that are arranged in a configuration one hologram wide and one beside the next, either in a straight line or in some other configuration, e.g., a circular configuration in a circular mediumxe2x80x94see FIGS. 2A-2D, 3A, 3B, 4A, and 4B. Perfect alignment is where the centers of the first and second set of holograms have the same position.)
This offset distance is typically an integer multiple of the spatial multiplexing shift desired in the completed recording medium, as discussed in more detail below. By forming a set of substantially non-overlapping holograms, the background for storage of subsequent sets is relatively stable, compared to more conventional storage techniques, e.g., polymeric shrinkage has taken place relatively uniformly across the row.
Where more than one row of holograms is desired, according to another aspect of the invention, first and second sets of holograms (or additional sets) are stored as shown in FIGS. 2A and 2B. Then, as shown in FIG. 2C, a third set (30, 32) of holograms are formed in a second, substantially planar row adjacent to the first row. Specifically, the third set is arranged such that the center of each hologram (30) is approximately one hologram diameter from adjacent holograms of the third set (32), and also approximately one hologram diameter from the center of an adjacent hologram of the first set (10). The fourth set (40, 42) is then also stored in the second row (with the centers of the holograms also spaced one hologram diameter apart). But, like the second set, the fourth set (40, 42) is shifted from perfect alignment with the third set of holograms (30, 32) by the same offset distance. Again, it is possible to form additional sets in each row, each set similarly shifted from perfect alignment with the previous set by the offset distance.
While the storage of sequential sets of edge-to-edge holograms has been discussed in other contexts, there has been no disclosure or motivation to use such a technique with photopolymer media. Instead, through careful analysis of the problems inherent in photopolymer media, it was discovered that a layered storage approach would be advantageous.
Specifically, the Ph.D. thesis xe2x80x9cHolographic 3-D Disks and Optical Correlators using Photopolymer Materialsxe2x80x9d by Allen Pu from the California Institute of Technology discusses storage of holograms in overlapping rows in lithium niobate. Careful consideration of the thesis shows that there would have been no motivation to extend Pu""s approach to photopolymer media. Chapter 3.4 of the thesis (pages 143-178) describes an experiment in which storage of 100 bits/xcexcm2 was being sought. As noted on page 144, because a suitable photopolymer medium was not available, lithium niobatexe2x80x94a photorefractive materialxe2x80x94was used instead. Initial experiments, as discussed at pages 156-158, encountered problems with the shift holography technique illustrated in FIG. 3.59(a)xe2x80x94the process of recording sequentially overlapping holograms in the photorefractive storage material caused at least partial erasure of previously-recorded holograms. Specifically, storage of each individual hologram decayed the strength of holograms previously recorded in that space (see page 156), and did so in a non-uniform manner. This problem detrimentally reduced the signal to noise ratio, particularly for the holograms in which erasure was most prevalent.
To address this erasure problem, which is unique to photorefractive materials, the thesis reports a particular storage technique, illustrated in FIG. 3.60(a). The technique involved recording sets of non-overlapping holograms in a row, such that an overlapping array as represented in FIG. 3.59(a) is ultimately achieved. This arrangement, by avoiding sequential overlap, allowed attainment of a more uniform erasure. However, erasure still occurred, and to reach similar diffraction efficiencies for all the holograms, the first holograms were recorded at the highest strength, with each subsequent set recorded at a lower strength.
As is apparent from the thesis and the above discussion, Pu was addressing an erasure problem unique to photorefractive materials. No such erasure occurs in photopolymer media. Thus, there would have been no reason to carry over this technique, which would be considered unnecessarily complex, to systems using photopolymer media.